Backward wave tube amplifieroscillator



BACKWARD WAVE TUBE AMPLIFIER-OSCILLATOR Filed Jan. 8, 1958 RF RF 1 IN OUT INVENTOR.

GE RALD KLEIN ATTORNEY nited States BACKWARD WAVE TUBE AMPLIFIER- OSCILLATOR Gerald Klein, Neptune, N.J., assignor to the United States of America as represented by the Secretary of the Almy Filed Jan. 8, 1958, Ser. No. 707,834

3 Claims. (Cl. 315--3.6)

(Granted under Title 35, US. Code (1952), sec. 266) atent Patented Jan. 31, rear unmodulated into a section of the helix with strong R-F plifiers and oscillators utilizing single periodic structures crossed-field type tubes are operated at a beam current which is below that required to initiate oscillations, it is possible to operate the tube as a backward wave amplifier which exhibits a selective gain-versus-frequency characteristic whose center frequency can be electronically tuned. However, appreciable gain can be achieved only if the tube is operated at a beam current which is quite close to the start-oscillation current. The gain is dependent upon the ratio of operating beam current, I to startoscillation current, 1,, and rises rapidly to infinity as this ratio approaches unity. Also, since the periodic slowwave structure or retardation line is a relatively lossless transmission line and forms a direct path between input and output, signals which are not amplified by the tube are coupled through with relatively low attenuation.

Therefore, the only discrimination between wanted and unwanted signals comes about as the result of the gain of the tube, and in the single periodic circuit tube this gain will be relatively low at values of current which are low enough to be safe from oscillations. One type tube designed to overcome the above mentioned limitations is known as the cascade backward wave amplifier and is described in the Proceedings of the IRE, November 1955,

pages 1617-1631. This cascade backward wave amplifier is an 0 type backward wave tube utilizing two helices through which the beam passes in successive order. The helices are dimensioned togive interaction with the first backward Wave harmonic, and the point of operation is chosen so that the beam current has a value just below the current necessary to start oscillations. The two helices are of the same length for optimum conditions. The input signal is applied to the collector end of the first helix, travels toward the gun and is amplified, and finally ab sorbed by a dissipating material at the gun end. The output signal builds up through backward interaction along the second helix and is extracted at the gun end. The collector end of the second helix is matched with a dissipative material to prevent reflections. The function of the first, or input helix is essentially to act as a modulator. When the beam passes through the second helix, a higher amount of energy may be extracted from the beam than in the single helix backward wave amplifier where the beam enters fields. The second helix acts as a demodulator. It has been found, however, that such a tube could not be employed as an oscillator since both sections have the same value of start-oscillation current, 1,, and oscillations in the gun end of the tube (input) would result in signals being generated and dissipated in the attenuator associated with the input section. This of course would greatly decrease the oscillatory output of the tube at the collector end (output). When operated as conventional backward wave oscillators, that is, when the beam current is above 1 the use of beam-type tubes is limited to low power output devices such as local oscillators in radar receivers. The crossed or transverse field backward wave travelling-wave tube, on the other hand, will provide high power oscil latory energy. 1

It is therefore a specific object of the present invention to provide a travelling-wave tube which combines the features of a high power electronically tuneable oscillator and a low level electronically tuneable selective amplifier.

It is yet another object of the present invention to provide a two-crcuit crossed-field backward wave oscillatoramplifier wherein good correlation exists between the frequency of maximum amplification and the frequency of oscillation.

It is still a further object of the present invention to provide a two-circuit crossed-field backward wave oscillatoramplifier which will provide high orders of discrimination between olf-freqency and on-frequency signals.

In accordance with the present invention there is provided a backward wave type crossed-field travelling wave tube which, for a prescribed electrostatic field, may be made to function either as an amplifier or as an oscillator at the same microwave frequency by merely varying the beam current. It includes a first retardation line or slow wave periodic structure of given length, and a second re-. tardation line, whose length is greater than the first line by at least a factor of 2, longitudinally spaced from each other. Also included is a sole plate or conductor situated parallel to the retardation lines and defining therewith an interaction space, and means for establishing the prescribed electric field between both retardation lines and the sole plate. In addition, there is included means for establishing a magnetic field perpendicular to the electric field. The distal ends of the retardation lines are terminated by respective matched loads and the proximal ends thereof are coupled respectively to input and output microwave frequency terminals. Further included are a collector anode situated near the matched termination of the second retardation line, means comprising an emissive cathode situated near the matched termination of the first retardation line for producing a beam of electrons directed towards the collector anode perpendicular to the magnetic and electric fields, and means in circuit with the cathode for varying the electron beam current. For an electron beam current below the start-oscillation current of the first or input retardation line, microwave energy at a given frequency introduced into the input line will be amplified and extracted from the output terminal of the second or ouput retardation line. With the same prescribed electric field and for a beam current substantially three times that of the start-oscillation current of the output line, oscillation energy at substantially the same given frequency will be extracted from the output terminal of the second line. The first line will not alfect the oscillatory energy since the beam current in the input section will be below the start-oscillation current thereof.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing which shows one embodiment of the invention.

astazu Referring now to the drawing, the tube elements are enclosed in metal envelope 10 and comprise, in the usual manner, an electron gun at one end thereof and a collector anode 14 at the other end thereof. The electron gun includes an accelerating electrode 16 and a cathode 1-8. Two slow-wave periodic structures or retardation lines, 22 and 24, of similar construction but of unequal lengths are linearly aligned and longitudinally spaced from each other. Although, the slowwave periodic structures are shown as conventional inter-digital lines with both sets of teeth extending from opposing walls of envelope 10, any other suitable periodic structure well known in the art may be used. Parallel to both periodic structures is an electrode 26, commonly known in the art as a sole plate. An electric field between periodic structures 22 and 24 and sole plate 26 is set up by means of source 23 having a negative potential applied to sole plate 26 and the positive potetnial of the source 28 shown connected to ground as is collector anode 14 and envelope 10. The beam current of the tube may be varied by changing the positions 'of voltage tap-offs 29 and 30 provided by rheostat 31 connected in parallel with source 23. The direction of the electric field established between the periodic structures 22 and 24 and the sole plate 26 by means of source 28 is as shown by the arrow. The shorter slow-wave periodic structure 22 of length L, hereinafter referred to as the input section, is placed near the cathode end of the tube and the longer periodic structure of length KL, hereinafter referred to as the output section, is placed near the anode end of the tube. As shown, the terminal of the input section 22 closest to the cathode is terminated in a matched load 30 and the other terminal thereof is connected to a conductor 32 to which is applied the input microwave energy. Similarly, the terminal of the output section 24 closest to collector anode 14 is terminated by a matched load 34 while the other terminal thereof is connected to output conductor 36. It is to be understood, of course, that matched loads 30 and 34 and input and output conductors 32 and 36 may be connected to the periodic structures by suitable microwave transition sections well known in the art. The electron beam 38 emitted from cathode 18 of electron gun 12 is propagated in a direction parallel to the periodic structures 22 and 24 in the interaction space between the periodic structures and sole plate 26, the beam 38 being in coupling relationship with both periodic sturctures. The interaction space between the periodic structures 22 and 24 and sole plate 26 is traversed by a magnetic field perpendicular to the drawing, the lines of force of which have been indicated at 40 and the strength of which will be determined by the known laws concerning the operation of tubes having crossed electric and magnetic fields. In accordance with invention, the lengths of the two periodic slow-wave structures are chosen such that the length ratio K is between 2 and 3, and preferably 2.5.

In discussing the operation of tube 10 as an amplifieroscillator, it is to be assumed that the sole plate 26 is at a prescribed negative potential with respect to the input and output periodic sections 22 and 24 to provide the electric field E and that the crossed-magnetic field is a value B For the input section 22, the start-oscillation current I may be expressed as 1 6 E A T where E V electron beam velocity E =D.-C. electric field A=cr0ss-sectional area of the beam w=frequency of input X21r L=length of input section e=permittivity of free space Similarly, for the output section of length KL, the start:

oscillation current I may be expressed as n l 6E 14. Iw QKZH Since in Equation 2, the values V E A and w are the same as that of Equation 1, then si. 2 I K (3) Equations 1 and 2 show that the start-oscillation current of each section is inversely proportional to the square of the length of the respective sections. Thus the output section will have the lowest start-oscillation current.

Assuming that the beam current is initially set at a value below the start-oscillation current of both the input section 22 and that of the output section 24, then signal frequencies applied to input conductor 22 which correspond to waves whose phase velocities are close to the electron velocity will be amplified and no oscillatory energy will be present. The input signal is amplified as it travels toward the gun end of the tube by interaction of the beam and the propagated wave in accordance with well known backward Wave tube amplification theory. The electron beam will be modulated by this interaction and pass through the output section 24 in coupling rela tionship therewith, and allow interaction with a back ward wave. The amplified output signal is then taken from the output conductor 36 of output section 24. Signals of other frequencies will not be coupled to the beam and will be absorbed in matched load 30. These signals will be decoupled from the output by the cold loss that can be obtained between input and output conductors 32 and 36. It has been found that with normal gain in the order of 5-10 db, the total discrimination between amplified and non-amplified signals is greater than 70 db.

Now, with the electron beam 38 increased to approximately three times that of the value of the start-oscillation current, I f output section 24 by means of accelerating electrode 16, but with the line-to-sole voltage unchanged, it was found that the output section 24 functioned as a normal high power crossed-field backward wave oscillator and that there was substantially no oscillation energy in the input section. The frequency of oscillation energy derived from output conductor 36 of output section 24 was found to be approximately equal to that of the frequency of maximum amplification for the same line-to-sole direct-current voltage field, E Hence, good correlation was found to exist between the frequency of maximum amplification and the frequency of oscillation for a fixed value of line-to-sole voltage. Since the oscillator portion of the tube must be operated at beam currents at least three times the value of 1 it can be seen from Equation 3 that K must be greater than /3 in order to insure that I do not operate above the current level I However, it is obvious that K cannot be made too close to /3 since random fluctuations ip tlie line-to-sole-voltage and beam current would cause a high degree of intermittent oscillatory action in the input section when the beam current is equal to 3 1 It was found that the value K =2 provided an adequate safety factor against such intermittent oscillatory action. Still further, it was found that optimum tube performance was obtained for K=2.5.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In backward-wave travelling-wave tube having crossed electric and magnetic fields and including a collector anode and means for directing an electron beam towards the collector anode in a direction perpendicular to the electric and magnetic fields at prescribed values of current; a first retardation line of given length and a second retardation line greater than said given length longitudinally spaced from each other and in coupling relationship with said electron beam, the first line being situated near the electron producing means and the second line being situated near the collector anode, said first and second lines having discrete start-oscillation currents, a sole plate parallel to said retardation lines and defining therewith an interaction space, said electric field being provided between said lines and said sole plate, the respective distal ends of said retardation lines being terminated by discrete matched loads and the respective proximal ends of said retardation lines being coupled to discrete microwave frequency energy terminals, the microwave frequency energy terminal of said first line being the input to said tube for introducing microwave energy into said first line and the microwave frequency energy terminal of said second line being the output to said tube for extracting microwave energy, the ratio of the lengths of said retardation lines being greater than /3 such that for a given electron beam current below the start-oscillation current for said first and second lines, microwave energy at a given frequency being introduced into the tube will be amplified and the amplified output will be available for extraction from said second line through its associated wherein the second retardation line is 2.5 times as long as said first retardation line.

References Cited in the file of this patent UNITED STATES PATENTS 2,794,936 Huber June 4, 1957 2,880,356 Charles et a1 Mar. 31, 1959 2,911,558 Currie Nov. 3, 1959 2,916,658 Currie Dec. 8, 1959 FOREIGN PATENTS 712,565 Great Britain July 28, 1954 766,724 Great Britain Jan. 23, 1957 OTHER REFERENCES The Cascade Backward-Wave Amplifier: A High- Gain Voltage Tuned Filter for Micro-waves, by M. R. Currie et al.,' Proceedings of the IRE, vol. 43, No. 11, November 1955, pp. 1617-1631. 

